In order to bore holes into the earth to investigate geological structures, it is usual to utilize a drill string that carries a drill bit at one end. The drill string is made up of a number of drill rods threaded together with each of the drill rods typically being in the order of ten feet in length. In order to drill the hole, the drill rod must be rotated and this is typically performed by a drill rig that allows the rods to be assembled end to end and also rotates.
Rotation is imparted to the rods by a drill chuck that engages the outer surface of the rod and transmits torque from a drive unit to the rod. The position of the chuck on the rod must be adjustable so that as the string penetrates the ground, the chuck can be released, moved along the rod, and re-engaged. These operations are under control of the driller who utilizes considerable expertise to maintain the rate of drilling as high as possible.
One of the principle factors affecting the drill penetration rate is the rotational speed of the drill string. The rotational rate of the drill string is governed in part by the capacity of the bearings that support the chuck and allow it to rotate. Generally speaking, the larger the diameter of the bearing, the lower its rated speed and therefore it is important to provide a chuck in which the supporting bearings are maintained at a minimum diameter.
Conventionally, the chucks are moved between open and closed positions utilizing a hydraulic actuator that operates on the jaws incorporated in the chuck to engage and release the drill rod. The hydraulic actuator is maintained stationary while the jaw assembly rotates with the drill string and bearings are therefore provided to permit this relative rotation. Conventionally, these are the principal bearings used to support the chuck for rotation and therefore it is their diameter that limits the rotational speed of the chuck.
The chucks operate in an adverse environment where they are likely to be exposed to mud, water and abrasive dust together with extremes of temperature. It is therefore important to provide an optimum environment for the bearings to avoid premature failure. Many arrangements have been proposed for incorporating the hydraulic motor into the bearing arrangement but these have tended to result in arrangements in which the bearing diameter is increased and the bearings are left exposed to the environment.